Transcript Document

Chapter 5: Other Major Current Systems
Key Points: Summary of Chapter 5
Components of the equatorial current system include:
• westward flowing N,S equatorial currents (driven by trades and geostrophy)
• eastward flowing counter current (surface/subsurface), and undercurrent
Equatorial Current System best defined in the Pacific (basin size, ITCZ)
ITCZ is north of the equator, as SE Trades cross hemispheres create
divergence just south of the equator and convergence around 4o N
Prevailing easterly winds slope the sea surface up to the western end of the
basin, creates an eastward directed pressure gradient, counter currents flow
eastward in regions of low wind stress (doldrums)
This pressure gradient force also drives the Equatorial Undercurrent (wind
stress at the surface around the equator is strong but at depth, baroclinic
conditions are sufficiently strong to drive the fast flowing current to the east)
Coriolis constrains the flow to the equator (meanders can exist)
Key Points Continued: Summary of Chapter 5
In the Pacific and Atlantic surface divergence south of the equator associated
with the South Equatorial Current (SEC) produces regions of upwelling.
Upwelling is also produced by Trade Winds blowing along the eastern side of
the basins. This can be seasonal in regions where the ITCZ migrates
The Asian Monsoons influence the circulation of the Indian Ocean. The
Equatorial Undercurrent is seasonal, and the surface Somali Current reverses
direction becoming an intense western boundary current during the
Southwest Monsoon. The North Equatorial Current also reverses direction
and become the South-West Monsoon Current
Disturbances in the ocean (such as Monsoons, ENSO, etc..) in part, propagate
as Kelvin and Rossby waves both at the surface (barotropic waves) and along
density boundaries (baroclinic waves)
Kelvin waves travel eastward along the equator or along the coast (NH coast
on right side, SH coast on left side)
Rossby waves (Planetary waves, conservation of potential vorticity) travel
westward along lines of latitude (slower)
Keep In Mind:
Coriolis Force on the Equator is zero
Coriolis Force by 0.5o influences flow of water
Currents To Know:
North Equatorial Current (NEC)
South Equatorial Current (SEC)
Equatorial Counter Current (ECC)
Equatorial Under Current (EUC)
Migration of the ITCZ
•NE and SE Trade Winds cross the equator
•Seasonal
•Variations in the magnitude and presence
of equatorial currents, in each basin
•Equatorial current system best defined in
the Pacific Ocean (trade winds blow over
longer distances, less migration of the ITCZ)
Refined Picture of the Equatorial Current System: Improved Measuring Techniques
Variability in
magnitude
NEC, SEC are driven by wind and geostrophy
NECC, pressure gradient, Coriolis adds to the convergence at 4o N
associated with the SEC
NECC, SECC flow in regions of weak winds ‘doldrums’
Subtropical counter-currents -------- horse latitudes?
Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient
Wind driven water from the surface mixed layer piled on the western side of the basin
Wind stress balances the pressure gradient (Coriolis Force ~= 0 at equator)
Adjustment (depression) of the thermocline on the western end
Baroclinic conditions at depth drive a jet-like current eastward eventually balance by
friction (eddy viscosity)
Equatorial Undercurrent (EUC) & Eastward Directed Pressure Gradient
Core of EUC around 200m water depth
Velocity can approach 1.5 m/s
ECC
EUC
North Subsurface Counter Current
westward
Think About Recirculation in the Equatorial Current System
Cromwell current (EUC) water originates primarily from the SEC
Once reaching the eastern basin EUC water feeds the NEC and the SEC
‘doming’ isotherms
Thermocline: 17o C
Thermostads - pycnostads
Remember:
sea surface slope – pressure gradient in the
surface mixed layer is opposite the slope of the
thermocline
Upwelling In Low Latitudes: Eastern Tropical Atlantic
Seasonal variation, migration of ITCZ, strength of SE Trades
More permanent upwelling regions associated with westward directed
wind stress
Monsoonal Circulation: winds that change seasonally
Northeast Monsoon in winter
• surface circulation of the northern
Indian Ocean most closely resembles
that of the Atlantic and Pacific
Southwest Monsoon in summer
• stronger of the two
Indian Ocean Surface Circulation
o
Somali Current 180 reversal
becomes an intense western
boundary current during southwest
monsoon.
Chlorophyll concentrations (orange/yellow): illustrating the effects of
upwelling associated with the southwest monsoon
Low level jet drives water away from the coast producing regions of
more intense upwelling as well as the fast flowing Somali Current.
Note the Somali Current is an intense western boundary current
but is not derived from the same balance of forces as the Gulf
Stream.
Agulhas Current
Agulhas – Benguela system
Agulhas Current: is the next most powerful western boundary current, second
only to the Gulf Stream. Its retroflection off the tip of southern Africa is a source
of eddies, many of which are carried into the Atlantic.
Think about: basin land boundaries, strength of coriolis etc.
Waves: the ocean can respond to the winds
in distant places by means of large-scale
disturbances that travel as waves.
Barotropic: surface waves
Baroclinic: density surface (thermocline)
Rossby (Planetary Waves)
Kelvin
Examples of barotropic and baroclinic waves
propagating through the ocean
Most tides are barotropic ‘Kelvin’ waves
Think about what would happen if the wind
stress was dramatically reduced or changed
directions in the case of the Asian Monsoon
Kelvin Waves
Travel eastward along the equator as a
double wave ‘equatorial wave guide’
Travel along coasts (coast on right in the NH
and on the left in the SH)
Balance between pressure gradient force and
coriolis force.
Kelvin Waves
Surface equatorial kelvin waves travel ~200 m/s
Rossby radius of deformation L = c/f
•High latitudes smaller eddies closely trapped
to the coast (increase in planetary vorticity)
•Low latitudes larger radius
Kelvin waves in the thermocline can have dramatic effects,
particularly in low latitudes where the mixed surface layer is thin.
Northward migration of ITCZ in western Atlantic generates
disturbance that propagates eastward
Splits into two coastal Kelvin waves when hits the eastern
boundary
The region of the disturbance where the thermocline bulges
upward cold nutrient rich sub-thermocline water can reach the
surface
4-6 week travel time
Rossby Waves:
Propagate from east to west across basin
Travel along lines of latitude
Move slower than Kelvin waves
Conservation of Potential Vorticity
Example: Waves in the jet-stream
Modeled Propagation of Equatorial Kelvin Wave
At mid latitudes - western sides of ocean basins are more connected to mid ocean
disturbances because Rossby Waves can communicate the information
At the equator the ocean can respond quicker to disturbances because both Kelvin
and Rossby waves can propagate
ENSO: El Nino – Southern Oscillation
“Interest in the phenomenon of El Nino goes back to the mid-19th century but it
was the El Nino event of 1972-73 that stimulated large-scale research into
climatic fluctuations, which began to be seen as a result of the interaction
between atmosphere and ocean.”
El Nino events are perturbations of the ocean-atmosphere system
Disturbance – a depression in the thermocline
accompanied by a slight rise in sea-level propagates
eastwards along the Equator as a pulse or series of
pulses (Kelvin Waves)
SO Index and Multivariate ENSO Index
Atmospheric pressure at sea level, zonal and meridional winds, sea-surface temperature,
surface air temperature, and the overall cloudiness
Satellite altimetry data for the 1997-98 ENSO event
At the eastern boundary, the equatorial Kelvin waves
split into northward – and – southward traveling
coastal Kelvin waves
Some energy is reflected back as a Rossby wave/s
Circulation At High Latitudes
The Arctic Sea:
Relatively enclosed basin (connection to
the Pacific through the Bering Strait and to
the Atlantic through the Greenland and
Norwegian Seas)
Enclosed nature influences ice cover
Circulation: was originally deduced from ice
flows and drifting ships, supplemented with
direct current measurements and geostrophic
calculations
Seasonal change in ice cover for
northern and southern high
latitudes
~10% of Arctic ice leaves
annually through the Fram
Straits between Greenland and
Spitsbergen
The Great Salinity Anomaly
1973 – 1981
“The Day After Tomorrow” omg
Low salinity plume of water circulates
on the surface and disrupts
deepwater formation
Antarctic Circumpolar Wave (from Wikipedia)
The Antarctic Circumpolar Wave is a coupled ocean/atmosphere
wave that circles the Southern Ocean in approximately eight years.
Since it is a wave-2 phenomenon (there are two ridges and two
troughs in a latitude circle) at each fixed point in space a signal
with a period of four years is seen. The wave moves eastward with
the prevailing currents.
Note that although the "wave" is seen in temperature, atmospheric
pressure, sea ice and ocean height, the variations are hard to see in
the raw data and need to be filtered to become apparent. Because
the reliable record for the Southern Ocean is short (since the
early 1980s) and signal processing is needed to reveal its existence,
some climatologists doubt the existence of the wave. Others
accept its existence but say that it varies in strength over
decades.
The wave was discovered simultaneously by Warren White and R G
Peterson; and Jacobs and Mitchell; in 1996. Since then, ideas about
the wave structure and maintenance mechanisms have changed and
grown: by some accounts it is now to be considered as part of a
global ENSO wave.
References: White, W.B. and R.G. Peterson (1996): An Antarctic circumpolar wave in surface
pressure, temperature and sea-ice extent. Nature 380:699–702.
Jacobs, G. A., and J. L. Mitchell (1996): Ocean circulation variations associated with the
Antarctic Circumpolar Wave. Geophysical Research Letters 23(21):2947–50